Flat type heat pipe

ABSTRACT

A flat type heat pipe ( 10 ) is disclosed which includes a metal casing ( 12 ) and a wick structure ( 16 ) arranged inside the metal casing. The metal casing has an evaporating section ( 123 ) and a condensing section ( 124 ). The wick structure extends from the evaporating section towards the condensing section of the metal casing and has a first section in conformity with the condensing section of the metal casing and a second section in conformity with the evaporating section of the metal casing. The first section has a pore size larger than that of the second section of the wick structure. The wick structure includes a metal foam.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for transfer ordissipation of heat from heat-generating components, and moreparticularly to a flat type heat pipe applicable in electronic productssuch as personal computers for removing heat from electronic componentsinstalled therein.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their lowthermal resistance, and therefore are an effective means for transfer ordissipation of heat from heat sources. Currently, heat pipes are widelyused for removing heat from heat-generating components such as centralprocessing units (CPUs) of computers. A heat pipe is usually a vacuumcasing containing therein a working fluid, which is employed to carry,under phase transitions between liquid state and vapor state, thermalenergy from one section of the heat pipe (typically referring to as the“evaporating section”) to another section thereof (typically referringto as the “condensing section”). Preferably, a wick structure isprovided inside the heat pipe, lining an inner wall of the casing, fordrawing the working fluid back to the evaporating section after it iscondensed at the condensing section. The wick structure currentlyavailable for heat pipes includes fine grooves integrally formed at theinner wall of the casing, screen mesh or bundles of fiber inserted intothe casing and held against the inner wall thereof, or sintered powderscombined to the inner wall of the casing by sintering process.

In operation, the evaporating section of the heat pipe is maintained inthermal contact with a heat-generating component. The working fluidcontained at the evaporating section absorbs heat generated by theheat-generating component and then turns into vapor. Due to thedifference of vapor pressure between the two sections of the heat pipe,the generated vapor moves and carries the heat simultaneously towardsthe condensing section where the vapor is condensed into condensateafter releasing the heat into ambient environment by, for example, finsthermally contacting the condensing section. Due to the difference ofcapillary pressure developed by the wick structure between the twosections, the condensate is then brought back by the wick structure tothe evaporating section where it is again available for evaporation.

In order to draw the condensate back timely, the wick structure providedin the heat pipe is expected to provide a high capillary force andmeanwhile generate a low flow resistance for the condensate. Also, thewick structure is expected to provide a high permeability at thecondensing section of the heat pipe in order for the condensateresulting from the vapor in that section to enter into the wickstructure more easily. However, the wick structure provided in theconventional heat pipe generally has a uniform pore size distributionover its entire length. This uniform-type wick structure cannot satisfythese requirements. If the condensate is not timely brought back fromthe condensing section, the heat pipe will suffer a dry-out problem atthe evaporating section.

Therefore, it is desirable to provide a heat pipe with a wick structurewhich can draw the condensate back from its condensing section to itsevaporating section effectively and timely.

SUMMARY OF INVENTION

The present invention relates to a flat type heat pipe. The heat pipeincludes a metal casing and a wick structure arranged inside the metalcasing. The metal casing has an evaporating section and a condensingsection. The wick structure extends from the evaporating section towardsthe condensing section of the metal casing and has a first section inconformity with the condensing section of the metal casing and a secondsection in conformity with the evaporating section of the metal casing.The first section has a pore size larger than that of the second sectionof the wick structure.

In the heat pipe, the first section of the wick structure generates arelatively low resistance for the condensate as it flows in thecondensing section, and the second section of the wick structure isstill capable of maintaining a relatively high capillary force fordrawing the condensate back from the condensing section towards theevaporating section. Meanwhile, the condensate in the condensing sectionis capable of entering into the wick structure easily due to arelatively high permeability of the first section of the wick structure.As a result, the condensate is drawn back to the evaporating sectionrapidly and timely, thus preventing the potential dry-out problemoccurring at the evaporating section.

Other advantages and novel features of the present invention will becomemore apparent from the following detailed description of preferredembodiment when taken in conjunction with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a transverse cross-sectional view of a heat pipe in accordancewith a first embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view of the heat pipe of FIG.1, taken along line II-II thereof;

FIG. 3 is a transverse cross-sectional view of a heat pipe in accordancewith a second embodiment of the present invention;

FIG. 4 is a plan view of a portion of a metal casing of the heat pipe ofFIG. 3, showing an interior of the metal casing; and

FIG. 5 is a transverse cross-sectional view of a heat pipe in accordancewith a third embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a flat type heat pipe 10 in accordance with a firstembodiment of the present invention. The heat pipe 10 has a plate-typeconfiguration and includes a metal casing 12. The metal casing 12includes a top plate 121 and a bottom plate 122 cooperating with the topplate 121 to define a chamber 14 in the metal casing 12. A wickstructure 16 is provided inside the heat pipe 10, occupying a centralregion of the chamber 14. The wick structure 16 is so dimensioned as tofit between the top and bottom plates 121, 122 of the metal casing 12.The metal casing 12 is made of high thermally conductive material suchas copper or aluminum. The heat pipe 10 is evacuated and hermeticallysealed after a working fluid (not shown) is injected into the chamber 14of the metal casing 12. The working fluid is saturated in the wickstructure 16 and is usually selected from a liquid such as water oralcohol, which has a low boiling point and is compatible with the wickstructure 16. The wick structure 16 is a porous structure and is in theform of a metal foam.

As shown in FIG. 2, the metal casing 12 has an evaporating section 123and an opposing condensing section 124 along a longitudinal direction ofthe heat pipe 10. The evaporating and condensing sections 123, 124occupy two end portions of the heat pipe 10, respectively. Although itis not shown in the drawings, it is well known by those skilled in theart that two ends of the heat pipe 10 are sealed. The wick structure 16extends in the longitudinal direction of the heat pipe 10 and has a poresize that gradually increases from the evaporating section 123 towardsthe condensing section 124.

In operation, the evaporating section 123 of the heat pipe 10 is placedin thermal contact with a heat source (not shown), for example, acentral processing unit (CPU) of a computer, that needs to be cooled.The working fluid contained in the evaporating section 123 of the heatpipe 10 evaporates into vapor upon receiving the heat generated by theheat source. Then, the generated vapor moves, via the other region ofthe chamber 14 without being occupied by the wick structure 16, towardsthe condensing section 124 of the heat pipe 10. After the vapor releasesthe heat carried thereby and turns into condensate in the condensingsection 124, the condensate is brought back by the wick structure 16 tothe evaporating section 123 of the heat pipe 10 for being availableagain for evaporation.

In the present heat pipe 10, the capillary forces and the flowresistances generated by different sections of the wick structure 16 aredifferent. The general rule is that the larger a pore size a wickstructure has, the smaller a capillary force and the lower a flowresistance it provides. A first section of the wick structure 16 inconformity with the condensing section 124 of the heat pipe 10 has apore size larger than that of a second section of the wick structure 16in conformity with the evaporating section 123 of the heat pipe 10.Thus, the first section of the wick structure 16 generates a relativelylow resistance for the condensate as it flows in the condensing section124, and the second section of the wick structure 16 is still capable ofmaintaining a relatively high capillary force for drawing the condensateback from the condensing section 124 towards the evaporating section123. Meanwhile, the condensate resulting from the vapor in thecondensing section 124 is capable of entering into the wick structure 16easily due to a relatively high permeability of the first section of thewick structure 16. As a result, the condensate is drawn back to theevaporating section 123 rapidly and timely, thus preventing a potentialdry-out problem occurring at the evaporating section 123.

The metal foam used to form the wick structure 16 may be made of suchmaterials as stainless steel, copper, copper alloy, aluminum alloy andsilver. The wick structure 16 may be formed independently of the metalcasing 12 and then inserted into the metal casing 12. Typically, themetal foam of the wick structure 16 is fabricated by expanding andsolidifying a pool of liquid metal saturated with an inert gas underpressure. The porosity of the foam after solidification may be in a widerange, subject to the levels of pressure applied during the fabricationprocess. Electroforming is another typical method for fabricating themetal foam, which generally involves steps of providing one kind ofporous material such as polyurethane foam, then electrodepositing alayer of metal over the surface of the polyurethane foam and finallyheating the resulting product at a high temperature to get rid of thepolyurethane foam to thereby obtain the porous metal foam. Still anotherfabrication method for the metal foam, called die-casting process, isalso widely used, which generally includes steps of providing one kindof porous material such as polyurethane foam, filling ceramic slurryinto the pores of the porous polyurethane foam and then solidifying theceramic slurry therein, then heating the resulting product at a hightemperature to get rid of the polyurethane foam to obtain a matrix ofporous ceramic, thereafter filling metal slurry into the pores of theceramic matrix and finally getting rid of the ceramic material aftersolidification of the metal slurry to thereby obtain the porous metalfoam. However, there are still some other methods suitable forfabrication of the metal foam. Fox example, the metal foam can be madeby steps of filling a kind of bubble-generating material such asmetallic hydride into metal slurry to generate a large number of bubblesdistributing randomly throughout the metal slurry and solidifying themetal slurry to thereby obtain the metal foam with a plurality of porestherein.

FIG. 3 illustrates a flat type heat pipe 20 in accordance with a secondembodiment of the present invention. In addition to the wick structure16 that is in the form of a metal foam, the heat pipe 20 also includes aplurality of fine grooves 201 longitudinally defined in an inner surfaceof the casing 22. These grooves 201 altogether function as another wickstructure cooperating with the original wick structure 16 so as toobtain a higher capillary force inside the heat pipe 20. Furthermore,each of the grooves 201 may have a varying width throughout the heatpipe 20. As particularly shown in FIG. 4, each groove 201 has a widthgradually increasing from the evaporating section 223 towards thecondensing section 224 of the heat pipe 20. This particular design ofthe grooves 201 can reduce flow resistance to the condensate as it flowsin the condensing section 224 of the heat pipe 20.

FIG. 5 illustrates a flat type heat pipe 30 in accordance with a thirdembodiment of the present invention. In this embodiment, two wickstructures 16 are arranged inside the heat pipe 30 with each beinglocated near a sidewall of the heat pipe 30. Thus, the central region ofthe chamber of the heat pipe 30 functions as a vapor channel for passageof vapor generated inside the heat pipe 30 from the evaporating sectionto the condensing section.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. A flat type heat pipe comprising: a metal casing having anevaporating section and a condensing section; and a wick structure madeof a metal foam and arranged inside the metal casing, the wick structureextending from the evaporating section towards the condensing section ofthe metal casing and having a pore size gradually increasing from theevaporating section towards the condensing section of the metal casing.2. The heat pipe of claim 1, wherein the wick structure extends along alongitudinal direction of the heat pipe and occupies a central region ofan interior chamber defined in the metal casing.
 3. The heat pipe ofclaim 1, wherein the wick structure extends along a longitudinaldirection of the heat pipe and is located near a sidewall of the metalcasing.
 4. The heat pipe of claim 1, wherein the metal casing defines aplurality of grooves in an inner surface thereof.
 5. The heat pipe ofclaim 4, wherein the grooves extend along a longitudinal direction ofthe metal casing and at least one of the grooves have a width graduallyincreasing from the evaporating section towards the condensing sectionof the metal casing.
 6. A flat type heat pipe comprising: a metal casingincluding a top plate and a bottom plate cooperating with the top plateto define a chamber inside the metal casing, the metal casing having anevaporating section and a condensing section; and a wick structurelocated inside the casing and occupying a portion of the chamber, thewick structure having a first section in conformity with the condensingsection of the metal casing and a second section in conformity with theevaporating section of the metal casing; wherein the wick structure issandwiched between the top and bottom plates of the metal casing andsaid first section has a pore size larger than that of the secondsection of the wick structure.
 7. The heat pipe of claim 6, wherein thewick structure is in the form of a metal foam.
 8. The heat pipe of claim6, wherein the metal casing has a plurality of grooves formed in aninner surface thereof.
 9. The heat pipe of claim 8, wherein the grooveseach have a width gradually increasing from the evaporating sectiontowards the condensing section of the metal casing.
 10. The heat pipe ofclaim 6, wherein the wick structure occupies a central portion of thechamber.
 11. The heat pipe of claim 6, wherein the wick structureoccupies a side portion of the chamber.
 12. A heat pipe, comprising: anelongated casing having a flat plate, a plurality of grooves formed inan inner surface of the casing along a longitudinal direction thereof;and a metal foam received in the casing and extending along thelongitudinal direction thereof, the metal foam having a pore size whichis gradually increased along the longitudinal direction of the casing,wherein the grooves and the metal foam cooperate as a wick structure forthe heat pipe for moving a condensate in the heat pipe.
 13. The heatpipe of claim 12, wherein at least one of the grooves has a width whichis gradually increased along the longitudinal direction of the casing.